This invention relates in general to filters and in particular to a new and useful air cleaning filter having an air cleaning material comprising a porous substrate containing zinc oxide with metal salt which is capable of forming an ammine, added thereto.
Activated carbon is employed for cleaning breathing air from noxious gases, for example in gas mask filters and air conditioning filters. To obtain a satisfactory separation, the activated carbon is impregnated in various ways.
German Standard DIN No. 3181 determines the ranges of protection against the various noxious substances. For example, breathing filters with the identification letter A protect against organic gases, with the letter B against acid gases, with the letter E against sulfur dioxide, and with the letter K against ammonia. A breathing filter protecting, for example, against a mixture of B and K, requires a preparation filling making sure such a double protection. This is mostly obtained with two layers, one of a preparation against acid gases, and the other against alkaline gases (ammonia). Such filters have the disadvantage of being very voluminous, heavy and expensive and having a high resistance to air flow.
Standard DIN 3181 further determines the absorbing capacity of the filters and their respective test concentrations by introducing a filter classification as small (1), medium (2), and large (3).
German Pat. No. 530,381, teaches a sorbent composition for use in gas filters, particularly for breathing purposes, which is particularly effective against hydrogen sulfide and ammonia. That composition comprises activated carbon on which cuprous chloride is deposited. While the reaction with ammonia is then conventional, with hydrogen sulfide, hydrochloric acid forms as the final product which is intended to be absorbed by the activated carbon. The fixing capacity of activated carbon is reduced by the deposition of cyprous chloride. Because of this, the presence of hydrochloric acid becomes objectionable. Depending on the loading of the filter, hydrochloric acid may pass into the inhalation air. Thus, another poisonous substance would simply replace the initial substance that was to be removed by the filter.
U.S. Pat. No. 1,519,470 provides impregnated carbon as an air cleaning means suitable for gas filter types A and B. Metals and metal oxides, such as copper, cupric oxide, silver, silver oxide, and zinc oxide are indicated as impregnating agents. In this method, the carbon is treated with copper salts, specifically nitrates and sulfates. This is followed by heating to disintegrate the copper to its oxide.
The impreganted carbon of U.S. Pat. No. 1,519,470, however, is not suitable as an air cleaning agent or filter and thus for filters having a wide range of application.